The present invention relates to an apparatus for converting a video signal or the like into a digital signal and performing band compression based on a combination of intra-frame-coding processing and inter-frame-coding processing, which apparatus allows a recording/reproducing device to easily obtain a good reproduced image especially in the fast reproduction mode when an output signal from the apparatus is recorded on a tape by a helical scan scheme and is transmitted to the recording/reproducing device for reproducing the signal. In addition, the present invention relates to an apparatus which can record signals in a wide band, used for a high-definition TV or the like, for a long period of time.
Both a transmission method based on a combination of intra-frame and inter-frame coding and a transmission method using a variable length coding scheme have been used to transmit data. Of these methods, the method of performing transmission by performing band compression based on a combination of intra-frame-coding processing and inter-frame-coding processing is associated with a band compression technique, as disclosed in, e.g., "Digital compatible HD-TV Broadcast system", Woo Paik, IEEE Trans. on Broadcasting Vol. 36 No. 4 December 1990. Characteristics features of this technique will be described below. Referring to FIG. 59, a video signal input to an input terminal 11 is supplied to a subtracter 12 and a motion evaluation circuit 13. In the subtracter 12, subtraction processing (to be described later) is performed. An output from the subtracter 12 is input to a DCT (discrete cosine transformation) circuit 14. The DCT circuit 14 receives the data in units of blocks, each consisting of 8 pixels in the horizontal direction.times.8 pixels in the vertical direction (8.times.8 pixels=64 pixels), and outputs coefficients obtained by transforming a pixel array from a time axis region to a frequency region. Each coefficient is quantized by a quantizer 15. In this case, the quantizer 15 has 32 types of quantization tables. Each coefficient is quantized in accordance with a selected quantization table. Note that the quantization tables are arranged in the quantizer 15 to set the generation and transmission amounts of information within a predetermined range.
Coefficient data output from the quantizer 15 is zigzag-scanned from a low-frequency region to a high-frequency region in units of blocks and is input to a variable length encoder 16 to be converted into a variable length code constituted by a set of a zero coefficient count (run length) and a non-zero coefficient. Note that the encoder 16 is a variable length encoder designed to change the code length in accordance with the frequency of occurrence of Huffman codes and the like. The variable-length-coded data is input to a FIFO (fast-in/fast-out) circuit 17 to be read out at a predetermined rate, and is subsequently supplied to a multiplexer (not shown) on the next stage (designed to multiplex a control signal, audio data, sync data (SYNC), NMP (to be described later), and the like) through an output terminal 18. The data is then sent to a transmission path. The FIFO circuit 17 serves as a buffer for absorbing the difference between the generation and transmission amounts of codes. This difference is based on the fact that the output rate of the variable length encoder 16 is a variable rate and the transmission rate of the transmission path is a fixed rate.
An output from the quantizer 15 is input to an inverse quantizer 19 to be inversely quantized. An output from the inverse quantizer 19 is input to an inverse DCT circuit 20 to be restored to the original signal. This signal is input to a frame delay circuit 22 through an adder 21. An output from the frame delay circuit 22 is supplied to a motion compensation circuit 23 and the motion evaluation circuit 13. The motion evaluation circuit 13 compares the input signal from the input terminal 11 with the output signal from the motion compensation circuit 23 to detect the overall motion of a corresponding image, thus controlling the phase position of the signal output from the motion compensation circuit 23. If the image is a still image, compensation is performed to cause the current image and an image one frame ahead thereof to coincide with each other. An output from the motion compensation circuit 23 is supplied to the subtracter 12 through a switch 24 and is also fed back from the adder 21 to the frame delay circuit 22 through a switch 25.
The basic operation of the system will be described below. The basis operation of this system includes intra-frame-coding processing and inter-frame-coding processing. Intra-frame-coding processing is performed as follows. During this processing, both the switches 24 and 25 are kept off. A video signal input to the input terminal 11 is transformed from a time axis region to a frequency region by the DCT circuit 14 and is quantized by the quantizer 15. The quantized signal is variable-length-coded and is output to the transmission path through the FIFO circuit 17. The quantized signal is restored to the original signal by the inverse quantizer 19 and the inverse DCT circuit 20 and is delayed by the frame delay circuit 22. That is, intra-frame coding is equivalent to a process of directly converting the information of an input video signal into a variable length code. This intra-frame processing is performed at a proper period, e.g., for every scene change of an input video signal or in units of predetermined blocks. Periodic intra-frame processing will be described later.
Inter-frame-coding processing will be described next. When this processing is to be started, both the switches 24 and 25 are turned on. As a result, a signal corresponding to the difference between an input video signal and a video signal one frame ahead thereof is obtained by the subtracter 12. This difference signal is input to the DCT circuit 14 to be transformed from a time axis region to a frequency region. The signal is then quantized by the quantizer 15. In addition, since the difference signal and the video signal are added together by the adder 21 and the resultant signal is input to the frame delay circuit 22, a predictive video signal predicting the input video signal on which the difference signal is based is generated and input.
FIGS. 60(a) and 60(b) show line signals sent to the transmission path. The signals are obtained by performing intra-frame coding and inter-frame coding of a video signal as a high-definition television signal in the above-described manner. This line signal is a signal on the transmission path and is obtained by multiplexing a control signal, an audio signal, a sync signal (SYNC), a system control signal, an NMP, and the like. FIG. 60(a) shows the first line signal. FIG. 60(b) shows each of the second and subsequent line signals. If this video signal is obtained by intra-frame coding, a proper video signal can be obtained by performing inverse conversion of the video signal. If the video signal has undergone inter-frame coding, inverse conversion of the signal will only reproduce a difference signal. Therefore, if a video signal (or a predictive video signal) reproduced one frame before the current frame is added to this difference signal, a proper video signal can be reproduced.
According to the above-described system, the entire information of an intra-frame-coded signal is variable-length-coded, and signals subjected to inter-frame coding in the subsequent frames transmit difference information, thus realizing band compression.
The definitions of sets of pixels to be processed by the band compression system will be described below:
Block: A block is a 64-pixel area constituted by 8 pixels in the horizontal direction.times.8 pixels in the vertical direction. PA1 Super block: A super block is an area of a luminance signal constituted by 4 blocks in the horizontal direction and 2 blocks in the vertical direction. This area includes one block of a color difference signal U and one block of a color difference signal V. The image motion vectors obtained by the motion evaluation circuit 13 are set in units of super blocks. PA1 band compression means for forming a(where a is a positive integer)picture areas for a frame, for generating a refresh signal by intra-frame-coding a video signal at a period of f frames (where f is an integer .gtoreq.2) using an inter-frame-coded signal and an intra-frame-coded signal, for outputting said refresh signal, said inter-frame-code signal, and said intra-frame-coded signal of a picture areas, from a band compression encoder at a predetermined transmission sequence during normal transmission, for inputting the output refresh signal, inter-frame-coded signal, and intra-frame-coded signal to a band compression decoder, and then for obtaining a decoded picture; PA1 said video signal being formed of a set of successive pictures, said inter-frame-coded signal being formed by inter-frame-coding the video signal using a difference between a video signal of a present picture and that of a predicted picture, said intra-frame-coded signal being formed by intra-frame-coding the video signal using intra-frame information; and PA1 wherein said signal processor comprises a flag for indicating a special reproducing mode which includes a high-speed reproducing mode and a high-speed inverse reproducing mode, while recording and reproducing the decoded signal on a recording medium and transmitting the decoded signal. PA1 band compression means for forming a(a is a positive integer) picture areas for a frame, for generating a refresh signal by intra-frame-coding a video signal of b picture areas (b is an integer a&gt;b&gt;0) out of said a picture areas for a frame at a period of f frames (f is an integer.gtoreq.2) using an inter-frame-coded signal and an intra-frame-coded signal, for outputting said refresh signal, inter-frame-coded signal and intra-frame-coded signal of a picture areas, from a band compression encoder at a predetermined transmission sequence during normal transmission, inputting the output refresh signal, inter-frame-coded signal, and intra-frame-coded signal to a band compression decoder, and then for obtaining a decoded picture, said video signal being formed of a set of successive pictures, said inter-frame-coded signal being formed by inter-frame-coding the video signal using a difference between a video signal of a present picture and that of a predicted picture, said intra-frame-coded signal being formed by intra-frame-coding the video signal using intra-frame information, PA1 wherein said signal processor comprises a flag for indicating a special reproducing mode which includes high-speed reproducing mode and high-speed inverse reproducing mode, adds and records an address signal indicating a position of said refresh signal of b picture areas for a frame, on a frame, designates a special reproducing mode using the flag during high-speed reproducing process, determines the picture areas on the frame using said address signal, and displays the refresh signals of one of part of and all picture areas of the determined picture areas.
Macro-block: A macro-block is constituted by 11 super blocks in the horizontal direction. When codes are to be transmitted, DCT coefficients in a block are transformed into codes determined by continuous zero coefficient counts and the amplitudes of non-zero coefficients and are transmitted in sets. An end-of-block signal is added to the end portion of each block. Motion vectors obtained by motion compensation in units of super blocks are added and transmitted in units of macro-blocks.
Features especially associated with the transmission signals shown in FIGS. 60(a) and 60(b) will be described in more detail. The sync signal (SYNC) of the first line indicates a frame sync signal in a decoder. All the timing signals for the decoder are generated by using one sync signal per frame. The NMP signal of the first line indicates a video data count from the end of the first line signal to a macro-block of the next frame. Since codes are generated by adaptively switching intra-frame-coding processing and inter-frame-coding processing, the code amounts of frames differ from each other, and the positions of codes vary. For this reason, the NMP signal indicates the positions of codes corresponding to one frame.
In addition, periodical intra-frame processing is performed to cope with a case wherein a user changes a channel. In this band compression system, as described above, 11 super blocks in the horizontal direction are called a macro-block, and 44 super blocks are present in one frame in the horizontal direction. That is, 4 macro-blocks in the horizontal direction .times.60 macro-blocks in the vertical direction, i.e., a total of 240 macro-blocks, are present in one frame. In this band compression system, as shown in FIGS. 61(a) to 61(h) and 62(a) to 62(c), refreshing is performed for every vertical array of super blocks in units of 4 macro-blocks, and all the super blocks are refreshed at a period of 11 frames. That is, when the refreshed super blocks of 11 frames are accumulated, the intra-frame processing in all the areas is completed, as shown in FIG. 62(d). For this reason, in the normal reproduction mode of, e.g., a VTR (video tape recorder), the above-described intra-frame-coding processing is performed at a period of 11 frames, reproduced images can be watched without problems.
Note that head data is inserted in a start portion of each macro-block described above. This head data includes a collection of the motion vectors of the respective super blocks, field/frame determination data, PCM/DPCM determination data, quantization levels, and the like.
The above-described band compression system is used as an encoder for band compression of a television signal. At the receiving end, a corresponding decoder is used. Consider a case wherein the above-described transmission signal is recorded by a VTR. A general VTR employs a recording scheme in which a one-field video signal is converted into a fixed length code to generate a predetermined amount of information, and the information is recorded on X (X is a positive integer) tracks.
In contrast to this, if a transmission signal obtained by the band compression system is directly recorded/reproduced by the VTR, since a variable length code is used as a code processed by intra-frame coding and inter-frame coding, the position at which a code periodically intra-frame-coded is recorded is not fixed. Therefore, in the fast reproduction mode, blocks which are not refreshed are generated.
FIG. 63 shows track patterns obtained when the signal variable-length-coded in the above described manner is helically recorded on a magnetic tape 26. In track patterns T.sub.1 to T.sub.11, thick lines indicate positions where frames F.sub.1 to F.sub.11 are switched. The reason why the switching positions of the frames F.sub.1 to F.sub.11 are not aligned with each other is that recording data is prepared by variable length coding. In the normal reproduction mode of the VTR, since all the track patterns T.sub.1 to T.sub.11 of the magnetic tape 26 are sequentially scanned by a magnetic head, a proper video signal can be reproduced without problems by decoding the reproduction output using a decoder. That is, in the normal reproduction mode, all the codes processed by intra-frame coding and inter-frame coding and recorded on the magnetic tape 26 can be reproduced so that a proper image can be constructed by using all the codes.
In the VTR, however, only limited tracks are sometimes reproduced as in a double-speed reproduction mode as a special reproduction mode. In this mode, the magnetic head jumps over tracks to pick up recorded signals. In this case, if intra-frame-coded signals recorded on tracks are sequentially reproduced, no problems are posed. If, however, inter-frame-coded signals recorded on tracks are reproduced, only images reproduced by difference signals can be obtained.
FIGS. 64(a) and 64(b) show traces X.sub.1 to X.sub.11 of the magnetic head in the double-speed reproduction mode. Referring to FIGS. 64(a) and 64(b), since intra-frame-coded signals are separately recorded on frames F.sub.1 to F.sub.24, the position of an intra-frame-coded portion reproduced within a frame is indefinite. FIGS. 65(a) to 65(h) and 66(a) to 66(c) show intra-frame-coded signals which can be reproduced in the double-speed reproduction mode. When the signals of 11 frames are accumulated, as shown in FIG. 66(d), there are portions in which codes obtained by periodical intra-frame coding are not present, i.e., refreshed super blocks are not present, thus generating portions in which reproduced images cannot be constructed.